New insight into exchange energy of covalent chemical bonds

The exchange energy for the H-2 molecule is calculated for a wide range of internuclear distance R extending down to R = 0.5 a(0) using the surface in tegral method. This method accounts for the physical exchange of the two el ectrons between the two nuclei. The good agreement of the present results w ith the standard Heitler-London calculation demonstrates that the exchange energy can indeed be described in terms of electrons hopping back and forth between the nuclei, an interpretation which has been frequently criticized in the past.